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Re: alvarezsaur fingers

Vivian Allen <mrvivianallen@googlemail.com> wrote:

> In Crocodylians at least there is a large digital plantarflexor mainly
> associated with Digit 1 (the FDL-1, which to some extent is also
> attached to the main digital flexor tendon of FDL-2, but definitely
> sends a discrete tendon to the first digit - pers obs, but see Meers
> 2003, 'crocodylian forelimb musculature and its relevance to
> Archosauria'), that is roughly equivalent in mass to the main flexor
> for all the other digits.  In theory that would favour the pollux with
> a greater degree of individual control and ability to exert forces in
> isolation to the other digits, which all share their major
> plantarflexor. I dunno if that's the case with birds (I'm not familiar
> with their forelimb anatomy), but based on crocodylians, the pollux in
> archosaurs may have already been functionally distinct to some degree.
> Maybe a reason to keep that one and not the others?

Spinosaurids, compsognathids, _Nqwebasaurus_ and alvarezsaurids all
show an enlarged pollex.  That of _Nqwebasaurus_ is inferred to have
had some opposability to the other two digits when flexed.

Spinosaurids also have rather short forelimbs - but very massive
(especially the forearm, which also bears a huge olecranon process),
as occurs in other spinosauroids/megalosauroids.  It's interesting how
in several theropod groups enlargement of the pollex (and its claw)
goes together with rather short forelimbs.  Alvarezsaurids represent
an extreme condition.

The reason(s) why the first finger (pollex) was the one finger that
was kept by alvarezsaurids is a fascinating part of the story.  The
other part is why the alvarezsaurid manus became functionally
monodactyl in the first place.